Bryris,
Your ESC has no idea what type of battery is pushing current into it.
Yes, you can directly repalce a NiMh with a LiPo.

There are many good chargers out there - I am using a Triton II.
Hobby King has many that are very reasonably priced.
There are few that are both 12 VDC and 120 VAC, but they exist.
There is a new Triton that is.

Power supplies can be inexpensive, I use one from a flat screen monitor.
If going this route be carefull, some of them are > 15 volts and will destroy your charger.

Good Luck,
KW_Counter